The auto-balancing bridge method is one method for measuring the impedance used in the newest impedance measuring apparatuses. Measurement circuits that use the auto-balancing bridge method comprise a signal source for applying measurement signals to a device under test; a current-to-voltage converter for converting the current that flows through a DUT to voltage; and a vector ratio detector for measuring the voltage of measurement signals applied to the DUT and the output voltage of the current-to-voltage converter. The current-to-voltage converter that is used is one which is appropriate for the frequency of the measurement signals. Modem-type current-to-voltage converters are used in conventional measuring apparatuses in order to respond to a broad frequency range. It should be noted that modem means a modulation-demodulation system or modulator-demodulator. Modem-type current-to-voltage converters comprise a null detector, an quadrature detector, an integrator, and a vector modulator. Quadrature detector is referred as phase sensitive detector in below. Vector modulator is referred as vector generator in below. An auto-balancing bridge circuit that uses this modem-type current-to-voltage converter is called a modem-type auto-balancing bridge circuit or simply a modem-type auto-balancing bridge. See JP Unexamined Patent Application (Kokai) 2003-279,607 (pages 2 and 3, FIG. 7).
DC offset occurs in the phase sensitive detector and integrator of modem-type current-to-voltage converters. This DC offset is converted to AC by a vector generator, which is described later and produces an error in the impedance measurement values of the device under test. This difference is called open offset because it can be regarded as constant impedance when a device under test is not connected to the impedance measuring apparatus. An object of the present invention is to eliminate or reduce the effect of open offset on measurement values when measuring the impedance of a device under test.